Studies on interactions of human herpesvirus-6 (HHV-6) or human cytomegalovirus (HCMV) with human immunodeficiency virus type 1 (HIV-1) are of interest in view of the suggested relationship between HHV-6 or CMV infections and the pathogenesis of AIDS. HHV-6 and HIV-1 can productively infect CD4+T cells and coinfection accelerates cytopathic effects. Studies were conducted to understand how these interactions occur at the molecular level. We tested HHV-6 genes that may affect HIV-1 replication. By DNA transfection into human T-cells and monkey kidney cells, we have shown that an HHV-6 transforming gene segment, ZVH14, previously identified in this laboratory, can transactivate the HIV-1 LTR. This transactivation is mediated through Sp1 binding sites in the HIV-1 long terminal repeat. A 115-amino acid open reading frame, B115, in the ZVH14 DNA showed a similar transactivation capacity as ZVH14. In vitro expression studies found the B115 gene product of the predicted Mr = 18 kDa. Mutants of this protein were generated by site-specific mutagenesis to confirm its role in transactivation. A specific mutant having a deletion of 5 nucleotides from the 5' - end of the ORF showed significant reduction in transactivation of the HIV-1 LTR. This observation supports our finding that B115 encodes a transactivator protein. In vitro expressed protein or the extracts from B115 transfected Cos-7 (SV40 transformed monkey kidney cells) cells did not show any binding to the HIV-1 LTR in gel shift assays. In a follow up study, we have shown that B115 can also transactivate the CMV immediate early promoter in human fibroblasts and monkey kidney cells. Sp1 binding sites seem to mediate this transactivation as we have reported with the HIV-1 LTR activation. These data suggest that HHV-6 transforming gene segment, ZVH14, encodes a transactivator protein which can activate transcription from heterologous promoters and Sp1 binding sites are essential for this transactivation. This study is important to better understand the role of HHV-6 in the pathogenesis of AIDS and CMV retinitis. The project will also provide knowledge and tools for development of novel antiviral therapies and safe viral and plasmid vectors for gene therapy of AIDS.